Optical scanners are known in the art. In an optical scanner, a light source is directed toward a document while an array of photo detectors or charge coupled device (CCD) elements, which define imaging pixels, scan the document. The array detects light reflected from images or text on the document during a scanning pass. Digital data is generated based on the light detected by the array, which in turn is based on the image or text on the document. The digital data can be, for example, analyzed by optical character recognition (OCR) software to convert from raw digital pixel data or image data into text capable of being manipulated by a word processor, or can be imported into a graphics software program for image editing.
It is known to calibrate such scanners. For example, because of manufacturing non-uniformity, dust or contaminates, or other causes, system response for imaging pixels may not be the same from pixel to pixel. Compensation for pixel-to-pixel variation may be provided, for example, in the charge measuring process by multiplying the output value for each photo detector by a gain value and then adding an offset value. Various methods of calibrating scanners are known in the art. For example, one calibration process involves sampling imaging pixels in response to scanning of a target of known characteristics, such as a white, grey, or black sample and then calculating required gain and offset values. See, for example, U.S. Pat. No. 5,563,723 to Beaulieu et al., which is incorporated herein by reference.
U.S. Pat. No. 4,605,970 to Hawkins, which is incorporated herein by reference, discloses calibrating an optical document digitizer in which a calibration strip is mounted to a rotatable drum of the digitizer. The calibration strip has an optical pattern of contrasting light and dark areas. The optical pattern has plural discrete blocks. Each of the blocks have first and second orthogonal edges. The first edges are precisely aligned with the axis of the drum and provide a scanning reference line for a Y-coordinate axis and trigger scanning in the Y-direction.
U.S. Pat. No. 5,194,966 to Quardt, which is incorporated herein by reference, discloses a method for calibrating an optical scanner by repetitively scanning a calibration sheet that has lines at a forty-five degree angle to the path of the document through the scanner and that has a predetermined image dot count. The sheet is repetitively scanned until scanner threshold values are determined which satisfy predetermined relationships with the dot count.
Multi-function devices are known in the art. For example, the assignee of the present invention manufactures multi-function devices such as devices including both a scanner and a printer in a common housing. By providing scanning and printing capabilities, the device can also function as a copier. Some multi-function devices include a color scanner and a monochrome (e.g. black and white or greyscale) printer. This is often true when the printer of the multi-function device is a laser printer. In such systems, monochrome printouts of scanned color documents may not show possible calibration problems with the color scanner.